Surface-micromachined Devices Research Articles

Micro Device Modeling Method and Design for Manufacturability

The micro device designing flow begins with the mask design currently, which is not intuitive for designer and sometime made them confused. As a result, the manufacturability is hard to prove in advance with the traditional designing way. Especially for surface micro-machined device with multiple structural layers, the 2D mask design cannot ensure the fabrication of complex 3D device with high yields reliably. The rise of structural designing strategies gave a direction to change the traditional habit. On the other hand, the top-down method provides the advanced designing flow, which begins with the system level design. However, there still some key technologies to study further because of the novel design procedure. To improve the manufacturability of micro device, the designing flow based on design for manufacturability methodology is presented, which draw on the advantages and experience of the mechanical design method. The key technologies on the structural design level, the processes planning level and the fabricating level are introduced correspondingly. In addition, we present the framework to implement the method. DOI: http://dx.doi.org/10.11591/telkomnika.v11i8.3081

Non-uniform residual stress assembly for free-space optical systems

When structures fabricated using surface-micromachining contain non-uniform stresses, those structures will deflect vertically during the release step. In this paper, we show how those deflections can be harnessed, and optionally amplified, to assemble out-of-plane components. This method of assembly introduces no new materials or steps into the fabrication process. Structures can be raised to 90° with respect to the substrate, or, with the addition of mechanical interlocks, to any arbitrary angle. In addition to reporting on the assembly of several surface-micromachined optical device using this method, this paper shows a simple free-space optical system. The optical system operates in the plane of the substrate surface, but light is coupled into the system perpendicularly to the substrate surface.

Optimization of Comb-Driven Devices for Mechanical Testing of Polymeric Nanofibers Subjected to Large Deformations

Comb-driven electrostatic actuators applied to mechanical testing of nanostructures are usually designed by a ldquobrute-forcerdquo approach for maximum electrostatic-force output, which results in limited actuation range. This issue is more prevalent when testing soft nanofibers with large ductility. In this paper, the design considerations for a comb-driven platform for nanoscale mechanical testing of ductile nanofibers subjected to 50%, or larger, inelastic extensions are presented. The optimization carried out aimed at increasing the net-force output by comb drives with clamped-clamped tethers, which also improves on the accuracy in the calculation of the force that is applied onto the nanofiber specimens. At large actuator motions, tethers of low bending stiffness increased the net force applied to a nanofiber and provided better accuracy in the calculation of the applied force. On the contrary, at small actuator motions, the maximum net-force output by the comb drives increased with the axial tether stiffness due to the associated increase in the pull-in-instability voltage. The fabricated surface-micromachined devices enabled experiments with individual electrospun polyacrylonitrile nanofibers at a maximum force of 30 muN and extensions up to 60%. The force output calculated from the voltage input to the electrostatic devices was compared to direct measurements by an independent optical method. [2008-0252]

New release technique of a thick sacrificial layer and residue effects on novel half-coaxial transmission line filters

This paper presents a new release technique for efficient and complete removal of the thick sacrificial layer applicable to surface-micromachined devices and compares this with other conventional release methods. A fully surface-micromachined half-coaxial transmission line filter having a large air-filled gap of 100 µm in thickness is successfully demonstrated using the proposed release technique. The effects of the sacrificial layer residue on the RF responses of the filters, completed by a conventional oxygen plasma ashing process, are analyzed with the aid of x-ray photoelectron spectroscopy (XPS). Experiments show that the proposed new release technique makes it possible to completely remove the thick sacrificial layer, shorten the process time, increase the after-fabrication yield and improve the process reproducibility of the large air-filled gap filters compared to the conventional oxygen plasma ashing method.

Free-standing <inline-formula><math display="inline" overflow="scroll"><msub><mi mathvariant="normal">C</mi><mn>60</mn></msub></math></inline-formula> nanowire fabricated using <inline-formula><math display="inline" overflow="scroll"><mrow><mi mathvariant="normal">Xe</mi><msub><mi mathvariant="normal">F</mi><mn>2</mn></msub></mrow></math></inline-formula> sacrificial dry etching

We report a fabrication of doubly supported free-standing buckminsterfullerene (C60) nanowires onto single crystal silicon microstructures in order to demonstrate an integration of carbon nanomaterials to surface-micromachined microelectromechanical devices. By irradiating vacuum-deposited C60 film with an electron beam, polymerized C60 nanowires were patterned. Free-standing structures of the C60 nanowires were obtained by sacrificial etching of underlying silicon structures using XeF2 gas. The supporting 5-µm thick silicon structure was released using vapor hydrofluoric acid. The C60 nanowire of 2-µm width, 25-µm length, and 40-nm thickniss connected to a movable single crystal silicon structure was successfully fabricated.

Release processing effects on laser repair of stiction-failed microcantilevers

Undesirable adhesion in microelectromechanical systems (MEMS) is referred to as stiction and is a principal failure mechanism in surface-micromachined MEMS devices. Previous investigations demonstrated repairing stiction-failed polycrystalline silicon MEMS structures released from isopropyl alcohol (IPA) using Nd:YAG laser irradiation and predicting the laser repair process using a thermomechanical model. The current paper reports the effectiveness of the laser repair process and corresponding thermomechanical model predictions for microcantilevers that have failed during four release treatments: water, IPA, octadecyltrichlorosilane (OTS), and supercritical CO/sub 2/ drying. Model predictions and experimental measurements of the laser repair process are also provided for MEMS devices that failed due to contact during electrostatic actuation. The results indicate that the laser repair process is very effective for both failure modes and that the thermomechanical model predicts the laser repair of microcantilevers that failed during release much better than for microcantilevers that failed due to subsequent contact.

Systematic mask synthesis for surface micromachined microelectromechanical systems

In the context of designing surface-micromachined microelectromechanical systems (MEMS), there does not appear to be systematic means, with the exception of parametrized layout models, to generate the mask data after the geometric model of a MEMS device is refined through behavioral simulations. This paper focuses on automatically generating masks, given a geometric model of the MEMS device and the process sequence (referred to here as the inverse problem). This necessitates a systematic solution of the forward problem, which involves automatically generating a geometric model of the MEMS device given the masks. A systematic and implementation-independent framework for the geometric modeling of MEMS is presented in order to solve the forward and inverse problems for general surface-micromachined devices. In particular, the geometric problem of mask synthesis is reduced to a system of linear equations.

The reliability of microelectromechanical systems (MEMS) in shock environments

As a first step toward formulating guidelines for the design of dynamically reliable MEMS, we analyze the mechanical response of, and formulate failure criteria for, a large class of shock loaded microsystems. MEMS are modeled as microstructures attached to elastic substrates, and the shocks are modeled as pulses of acceleration applied to the substrate over a finite time duration. The relevant time scales in the analysis are the acoustic transit time, the time period of vibrations, and the duration of the applied shock load. For many MEMS structures and shock loads (with durations in the range 50-5000 /spl mu/s), the substrates respond as rigid bodies and are expected to be immune to stress-wave-induced damage. Time-domain criteria, obtained to distinguish between the impulse, resonant, and quasistatic responses of the microstructures, correlate well with the experimentally observed responses of different MEMS devices. The formulation of displacement-based and stress-based failure criteria is discussed, along with their sensitivity to the applied strain rate. A case study, in which these results are applied to evaluate the reliability of a packaged surface-micromachined device, is presented.

Surface Oxide Effects on Static Fatigue of Polysilicon MEMS

ABSTRACTStatic fatigue – crack growth causing delayed failure under constant stress and involving stress corrosion cracking – was investigated in polysilicon MEMS using two different surface-micromachined devices. One exploited residual tensile stresses to create stress concentrations at micromachined notches, and the other involved a single-edge notched beam specimen integrated with an electrostatic comb-drive microactuator. Tests with both devices revealed that polysilicon is not susceptible to static fatigue in humid environments. However, when a relatively thick (45 to 140 nm) surface oxide was thermally grown on the microactuator devices, the specimens demonstrated delayed fracture in a humid ambient, presumably due to static fatigue of the surface SiO2. The stress intensities at the resulting cracks in the SiO2were then sufficient to cause catastrophic crack propagation through the polysilicon specimens. The implications of our data on the issue of fatigue in polysilicon is discussed.

Laterally driven accelerometer fabricated in single crystalline silicon

The request for wireless sensor operations grows in medical and automotive applications. These sensors receive their energy and send their data by a telemetric unit. The wireless transferred energy restricts the power consumption of the sensor and signal processing to less than 3 mW. Therefore, the sensor has to be operated in open-loop. Furthermore, a main focus is directed to increase the sensitivity of the mechanical–electrical transducer. Considering both open-loop and sensitivity, the sensor has to be optimized by referring to the structure height. The way for realizing high structures, as described in this paper, is the micromachining of silicon wafers with a specified thickness. The superior mechanical properties of single crystalline silicon compared to electroplated metals or surface-micromachined devices confirm the use of silicon as sensor material. A laterally driven accelerometer is simulated, designed and fabricated comprising the technologies of deep reactive ion etching (DRIE) of silicon, silicon direct bonding (SDB) and chemical mechanical polishing (CMP). Characterization results confirm the performance of this new technology. The open-loop sensor, which was characterized, had a height of 50 μm with damping constant greater than 0.1.

Batch transfer of microstructures using flip-chip solder bonding

This paper describes a novel method for transfer and assembly of microstructures using sacrificial-layer micromachining and flip-chip bonding. The technique is performed at room temperature (cold weld) and at the back end of the process flow and may thus provide a commercially viable alternative to monolithic integration and costly hybrid packages. The transfer is achieved using break-away tethers and by cold welding (compression bonding) electroplated indium solder bumps to electroplated copper pads. Both high-aspect-ratio MEMS devices as well as surface-micromachined devices have been successfully transferred using this method with no observable misalignment between moving and stationary parts. The maximum tensile and shear stress the solder bond can withstand before failure is measured to be 11/spl plusmn/3 MPa and 9/spl plusmn/1 MPa, respectively. The contact resistance is measured to be of the order of 1.5 m/spl Omega/ for a 65 /spl mu/m/spl times/65 /spl mu/m/spl times/4-/spl mu/m indium bump.

Wafer-Level Strength and Fracture Toughness Testing of Surface-Micromachined MEMS Devices

AbstractDetermination of the mechanical properties of MEMS (microelectromechanical systems) materials is necessary for accurate device design and reliability prediction. This is most unambiguously performed using MEMS-fabricated test specimens and MEMS loading devices. We describe here a wafer-level technique for measuring the bend strength, fracture toughness, and tensile strength of MEMS materials. The bend strengths of surface-micromachined polysilicon, amorphous silicon, and polycrystalline 3C SiC are 5.1±1.0, 10.1±2.0, and 9.0±1.0 GPa, respectively. The fracture toughness of undoped and P-doped polysilicon is 1.2±0.2 MPa√m, and the tensile strength of polycrystalline 3C SiC is 3.2±1.2 GPa. These results include the first report of the mechanical strength of micromachined polycrystalline 3C SiC.

Deflection of surface-micromachined devices due to internal, homogeneous or gradient stresses

Devices surface-micromachined out of a thin film containing an internal stress will be deformed upon release. The stress of a film can be approximated in second order by a biaxial homogeneous stress superimposed on a linear biaxial stress gradient. In order to model and reduce the stress-induced deformation, the stress state of a thin film is evaluated with surface-micromachined test devices. The homogeneous stress component is measured with deflecting indicators, and the gradient component is measured from the curvature of cantilever beams. A finite element model of a cantilever beam is developed and the parameters influencing the stress-induced deflection are investigated. The model is then extended to calculate the deflection of a doubly supported bridge and the result is compared with experiment. Finally, by using the finite element model of the bridge, the importance of the supports is demonstrated, indicating that reducing the undercut of the supports is an effective method to reduce the stress-induced deflection.

A novel deposition technique for fluorocarbon films and its applications for bulk- and surface-micromachined devices

Several deposition techniques and the characteristics of fluorocarbon (FC) films have been investigated. First, FC film is spin-coated from cyclized perfluoro polymer (CPFP), commercialized as CYTOP. Because of its excellent durability for alkalis or acids, the FC film is used for protection of the circuit area of an integrated accelerometer from a silicon alkaline etching process by 25 wt.% TMAH at 90 °C. Secondly, FC films are formed on a field-free zone with a capacitive-coupling-type plasma-polymerization equipment using perfluorinated liquids (C 7F 16, C 8F 18, etc.) as monomer species. The deposition rates and thermal stability of the films have been investigated. The FC films are effective for elimination of in-use stiction for surface-micromachined devices.

A pure CMOS surface-micromachined integrated accelerometer

A pure CMOS integrated accelerometer has been realized using surface micromachining as the structural technique. The samples are fabricated by a 14-mask 0.8 μm CMOS standard process in a Siemens production line. Only the standard layers of the process (350 nm polysilicon and 600 nm oxide as sacrificial layer) are used to build up the surface-micromachined device. Sensor release and antisticking are also CMOS-compatible. The movement of a seismic mass normal to the chip surface is capacitively detected (open loop) and transformed on chip into a digital output signal by a robust circuit for measuring sub-femtofarad capacitance difference. Parasitics are suppressed on chip. The sensor is designed to measure accelerations up to 50 g. A resolution of ± 0.6 g, corresponding to a capacitance change of ± 0.1fF, is observed.

Determining mean and gradient residual stresses in thin films using micromachined cantilevers

A technique that provides a first approximation to the mean, , and gradient, , components of residual stress in a thin-film material is discussed. In this method, measurements are made on a single micromachined cantilever, as opposed to an array of structures as used in the related critical-length buckling approach, to find tensile, compressive and gradient stresses. The measured deflection profile of a cantilever is reduced to rotation and curvature components, which are shown to derive independently from and , respectively. Essential to this method is the observation that a micromachined structure with a `nominally-clamped' boundary undergoes subtle rotation at its junction with the portion of the thin film that remains bonded to the substrate but is contiguous with the structure. This boundary rotation effect occurs through in-plane expansion or contraction of the bonded film following relief of residual stress. Thus, the deformation of the micromachined cantilevers considered here and of more general bulk- or surface-micromachined devices, can be strongly influenced by the state of stress in the still-bonded film.

Adhesion release and yield enhancement of microstructures using pulsed Lorentz forces

Adhesion of microstructures is an important failure mechanism in surface-micromachined devices. In this paper, a simple and effective method for releasing pinned microstructures is presented. The method uses the Lorentz force due to the interaction of a current with an external magnetic field to generate an upward force that frees the microstructures. The static and transient behavior of beams under the Lorentz force is examined. Critical values of current and pulse durations needed to release the microstructures are determined and verified with experimental data. Using this technique, previously pinned beams and rectangular plates have been released. The release technique is suitable for mass production environments since it is easily applied during the electrical testing of the device, thereby increasing the manufacturing yield.